This invention relates to the use of reactive polymeric surfactants in the formation of emulsions and further to the use of reactive polymeric surfactants in the formation of emulsions used to prepare microencapsulated products.
The reactive polymeric surfactants for use this invention (also referred to herein as polymeric stabilisers) generally have three moieties—a hydrophilic moiety, a hydrophobic moiety and a moiety that possesses reactive or cross-linking ability with respect to a monomer or prepolymer substance, or with respect to a selected ingredient in the dispersed phase of the emulsion. When these surfactants are used with a discontinuous phase dispersed in a predominantly aqueous continuous phase the hydrophobic moiety adsorbs strongly to the surface of the discontinuous phase while the hydrophilic moiety associates strongly with the aqueous phase, thereby conferring colloidal stability upon the discontinuous phase. The cross-linking moieties enable the surfactant to become reacted with or bound to the monomer or polymer or other ingredient as mentioned above, while the colloid stabilizing moieties of the surfactant provide surface active properties to the thus combined surfactant/monomer or polymer. Emulsions according to the invention may be used for a variety of purposes including the formulation of agrochemical active ingredients. As will be described below, the surfactants for use in this invention also provide a similar combining/surface active feature when used with emulsions used to prepare microencapsulated products.
The surfactants for use in this invention are selected from certain random graft copolymers and certain block copolymers. It should be noted that the random graft copolymers and block copolymers for use in the present invention are surfactants in their own right which are then bound at the emulsion interface by reaction of the cross-linking moiety.
Emulsions are important agricultural formulation types, for example as oil-in-water emulsions (EW's) or as components of suspo-emulsions (SE's) that comprise and EW and a suspension concentrate (SC). Emulsions may be become unstable for a number of reasons and may coalesce, flocculate or sediment. Such phenomena are undesirable and may complicate or prevent the use of the formulation. Thus considerable effort has been directed towards understanding how to induce stability by methods such as density matching, producing narrow particle size distributions to limit Ostwald ripening, and by using the optimum surfactants and colloid stabilisers. Ostwald ripening of emulsions may be promoted by surfactants that can carry the oil from droplet to droplet through the continuous phase, for example in micelles. Ostwald ripening may be prevented or inhibited by surfactants that do not form such carriers or that are fixed to the emulsion interface.
Conventional emulsion surfactants that are physically adsorbed at the interface between the discontinuous and continuous phases may be displaced by competitive desorption by other surface active agents that may be added to the formulation or by conditions that stress the formulation, for example temperature cycling and electrolyte concentration.
We have found that the stability of emulsions may be improved where the surfactants are fixed to the emulsion interface according to the present invention. Foaming is also eliminated when no free surfactant is available in the continuous phase.
Micro-encapsulation is a well-known technique used to prepare solid particles that contain enclosed within them a core comprising liquid and/or solid materials derived from a dispersed liquid emulsion phase. Thus the liquid core materials may be liquids per se, or liquids containing solids dissolved or suspended in them. Depending on the design of the microcapsule, the material enclosed will be released either in a slow controlled manner, or in a quick release. There are numerous well-known techniques for designing the contents and nature of the polymeric wall of the microcapsule to achieve these desired results. In the agricultural field, such microcapsules are used with pesticides such as herbicides, insecticides, fungicides, and bactericides, with plant growth regulators and with fertilizers. Non-agricultural uses include encapsulated dyes, inks, pharmaceuticals, flavouring agents and fragrances wherein the encapsulated medium is present as a liquid core.
The materials used in forming the walls of the microcapsules are typically resin intermediates or monomers. Various processes for microencapsulating material have been developed and described in the prior art. These processes can be divided into three broad categories—physical, phase separation and interfacial reaction methods. Phase separation and interfacial reaction methods generally proceed via the formation of an emulsion and are therefore of particular relevance to the process of the present invention.
In one process in the phase separation category, microcapsules are formed by emulsifying or dispersing the core material in an immiscible continuous phase in which the wall material is dissolved and then is caused to physically separate from the continuous phase, for example, by coacervation, and deposit around the core particles. In the interfacial reaction category, the core material is emulsified or dispersed in an immiscible continuous phase; then an interfacial polymerization reaction is caused to take place at the surface of the core particles, forming microcapsules.
Interfacial polymerization reaction methods have proven to be the most suitable processes for use in the agricultural industry for the microencapsulation of pesticides. There are various types of interfacial reaction techniques. In one type of interfacial condensation polymerization microencapsulation process, monomers from both the oil and aqueous phases are brought together at the oil/water interface where they react by condensation to form the microcapsule wall. In another type of polymerization reaction, the in situ interfacial condensation polymerization reaction, all of the wall-forming monomers or prepolymers are contained in the oil phase. The oil is then dispersed into a continuous aqueous phase comprising water and a surface-active agent. The organic phase is dispersed as discrete droplets throughout the aqueous phase by means of emulsification, with an interface between the discrete organic phase droplets and the surrounding continuous aqueous phase solution being formed. In situ condensation of the wall-forming materials and curing of the polymers at the organic-aqueous phase interface may be initiated by heating the emulsion to a temperature between of about 20° C. to about 100° C., optionally with adjustment of the pH. The heating occurs for a sufficient period of time to allow substantial completion of in situ condensation of the wall-forming monomers or prepolymers to convert the organic droplets to capsules consisting of solid permeable polymer shells enclosing the organic core materials.
One type of microcapsule prepared by in situ condensation and known in the art is exemplified in U.S. Pat. Nos. 4,956,129 and 5,332,584, which are incorporated herein by reference. These microcapsules, commonly termed “aminoplast” microcapsules, are prepared by the self-condensation of etherified urea-formaldehyde resins or prepolymers in which from about 50 to about 98% of the methylol groups have been etherified with a C4–C10 alcohol (preferably n-butanol). The prepolymer is added to or included in the organic phase of an oil/water emulsion. Self-condensation of the prepolymer takes place under the action of heat at low pH.
To form the microcapsules, the temperature of the emulsion is raised to a value of from about 20° C. to about 90° C., preferably from about 40° C. to about 90° C., most preferably from about 40° C. to about 60° C. Depending on the system, the pH value may be adjusted to an appropriate level. For the purpose of this invention a pH of about 2 is appropriate.
Another type of microcapsule prepared by in situ condensation and described in U.S. Pat. No. 4,285,720, incorporated herein by reference, is a polyurea microcapsule that involves the use of at least one polyisocyanate such as polymethylene polyphenyleneisocyanate (PMPPI) and/or tolylene diisocyanate (TDI) as the wall forming material. In the process described in this patent, the wall forming reaction is initiated by heating the emulsion to an elevated temperature at which point the isocyanate groups are hydrolyzed at the oil/water interface to form amines, which in turn react with unhydrolyzed isocyanate groups to form the polyurea microcapsule wall.
In another type of interfacial polymerization process, as mentioned above, wall-forming materials are contained in both the organic and aqueous phases of the emulsion. Such a process is described, for example, in U.S. Pat. No. 4,280,833 in which an isocyanate such as PMPPI is contained in the organic phase and a reactive amine such as hexamethylenediamine is contained in the aqueous phase. The two-wall forming materials react at the interface between the two phases to produce a polyurea microcapsule shell that contains or encloses materials to be encapsulated, which are themselves contained in the organic phase of the emulsion.
In most of these types of microencapsulation processes, external surfactants and/or other surface-active agents such as emulsifiers or colloid stabilizers, are employed. These materials, indeed, are employed in both the processes for preparing the microcapsules and in resulting formulations of them or produced from them. The emulsifier serves to reduce the surface tension between the oil and aqueous phases while the colloid stabiliser serves to ensure that the particles are kept apart. In the emulsification process the droplet size is largely controlled by the degree of applied shear and the type and amount of emulsifier employed. If there is significant competition between the emulsifier and the colloid stabiliser for adsorption to the oil droplet the colloid stabiliser may be displaced, leading to droplet coalescence. Following capsule formation, generally the same colloid stabiliser must stabilise a solid particle that has properties different from those of the starting oil droplet. If the colloid stabiliser is displaced from the capsule surface, the capsules may irreversibly agglomerate. Some protective colloids are disclosed, for instance, in U.S. Pat. Nos. 4,448,929 and 4,456,569, together with the patents mentioned above describing microencapsulation processes. Similarly, surface-active agents are needed in formulations made from the microcapsules. For example the products of typical microencapsulation processes are suspensions of the microcapsules in the aqueous phase (generally termed “capsule suspensions”). In some cases the capsule suspension will be packaged and sold as such. However, so doing requires the storage and transportation of substantial amounts of water or other liquid. Therefore, another technique is to produce a dried microcapsule product (for example by spray drying or film drying of the capsule suspension) and then sell the dried product, either as a powder or in another solid form, such as tablets, extruded granules, etc. In all these cases, the dried product is designed to be mixed with water to form a sprayable suspension of the microcapsules. In order for the sprayable suspension (whether prepared from a powder, granule, or other form of microcapsule) to be relatively uniform so as to provide a substantially uniformly effective product when sprayed, it is necessary that the microcapsules be well dispersible in water. The inclusion of surface-active agents in the product as sold, or the addition of surface-active agents to the applicator's spray tank or other spraying equipment is often necessary to accomplish this purpose.
In all of the above-mentioned uses of surface active agents, certain shortcomings or disadvantages may appear. A typical one is that a surface active agent may become disassociated from the particles with which it is meant to interact. This can occur in emulsion or emulsion formation, or in production or formulation of microcapsules. In such case, the effectiveness of the surface active agents is decreased or lost because the uniformity of particle size or particle dispersability is not attained.
The properties of these surface active agent materials are determined by the composition and quantity of their hydrophobic and hydrophilic components. Where the formulation comprises a discontinuous oil phase dispersed in an aqueous continuous phase, the hydrophobic components of the surface active agents must adsorb strongly to the surface of the discontinuous phase while the hydrophilic components of the surface active agents must afford colloidal stability, thereby preventing the discontinuous phase from agglomerating.
The compositions and methods of preparation of polymeric surfactants are many and varied. A review of such materials is given in the text by Piirma: Polymeric Surfactants, Surfactant Science Series 42, (Marcel Dekker, New York, 1992). The two main classes of polymeric surfactants are those prepared as hydrophilic-hydrophobic blocks and those prepared as combs of hydrophilic arms attached to a hydrophobic backbone, and vice versa. Such hydrophobic-hydrophilic polymers have been termed “amphipathic” or “amphiphilic”. Adsorption to the discontinuous phase is maximised where the surfactants have little or no propensity to micellise in the continuous phase.
In general, polymeric surfactants may be made by modifying previously prepared polymers or by polymerisation in a single step or stepwise manner. For example block co-polymers can be made by (i) the controlled stepwise polymerisation of firstly hydrophobic and secondly hydrophilic monomers, or the reverse of this process, or by (ii) coupling together pre-formed hydrophobic and hydrophilic materials of suitable molecular weight. Graft copolymers can be made by (i) graft polymerisation of hydrophilic monomers or macromonomers to a hydrophobic backbone, or the reverse of this process, or by (ii) chemically converting suitable monomers which have been co-polymerised with hydrophobic or hydrophilic backbone monomers. Polymers having similar surfactant properties to those of graft copolymers can be made by randomly copolymerising hydrophilic and hydrophobic monomers or hydrophilic macromers and hydrophobic macromers.
The preferred preparative method for any given composition will depend on the nature and properties of the starting materials. For example, the reactivity ratios between certain monomers may limit the amount of a hydrophilic monomer that can be radically co-polymerised with hydrophobic monomers.
Although non-reactive surfactants/emulsifiers and colloid stabilisers are very widely used, formulations made from these materials sometimes have disadvantages. For example these ‘conventional’ materials are adsorbed to the interface of the discontinuous phase simply by physical adhesive forces and, under certain circumstances, can be desorbed, thereby resulting in colloid instability. Surfactants which can form micelles may aid the transport of material from the colloid to the continuous phase. This may be undesirable if the transported material is able to change its physical state, for example by crystallization, in the continuous phase. The preparation of certain formulations such as microcapsules made by interfacial polymerisation methods may require high levels of surfactant which may adversely affect both processing and the properties of the capsules. Moreover high levels of ‘free’ surfactant may be washed off in the aqueous phase leading to undesirable environmental contamination.
The above disadvantages may be largely overcome if the surfactants become chemically or irreversibly bound to the surface of the discontinuous phase.
One example of this approach is disclosed in U.S. Pat. No. 5,925,464. As described in that patent, polyvinyl alcohol (PVA) is utilized during a microencapsulation process, preferably one between a polyisocyanate and a polyamine, and reacts with the isocyanate to incorporate polyurethane groups into the microcapsule walls. This enables the surface active properties of PVA to be bound to the microcapsules and, as stated in the patent, “to produce a uniform layer of water-soluble polymer around each capsule” which should film form when dried. PCT application WO 98/03065 discloses a similar concept using what is termed “a non-micellising surfactant” (however, the only surfactant disclosed in the text is PVA.)
It would be advantageous to provide surfactants that can become bound to the surface of such microcapsules or alternatively can become bound to emulsion droplets so that the surface-active action is maintained relatively uniformly throughout the resulting product or is available relatively uniformly during the process of preparing the product, for instance, processes for producing microcapsules.
In U.S. Pat. No. 6,262,152 (Fryd et al) there is disclosed a dispersion of particles such as pigments in a liquid vehicle in which the solid particle is insoluble wherein the particles are entrapped in a polymer matrix formed by cross-linking of moieties of a polymer dispersant having at least one segment soluble in the liquid vehicle and at least one segment insoluble in the liquid vehicle wherein said insoluble segment has the cross linkable moieties. The network or matrix of cross-linked polymer matrix which surrounds each particle is formed of very stable cross-linking bonds which effectively prevent the particle from leaving the “core” formed by the polymer. The Examples show that a relatively high proportion of polymer is required to provide this polymer matrix which surrounds and entraps the particle.